1. Field of the Invention
The present invention relates to flexible self-expandable stents made of super-elastic shape memory alloys and used for insertion in the desired parts of contracted muscular passages, contracted blood vessels, or arteries having an aneurysm so as to open the contracted parts or repair the arterial dilatation and, more particularly, to a flexible self-expandable stent, designed to maintain a shape thereof corresponding to the desired part of a contracted muscular passage, a contracted blood vessel, or an artery having an aneurysm, thus being effectively used for opening the contracted part or repairing the arterial dilatation, without deforming the shape of the contracted muscular passage, the contracted blood vessel, or the dilated artery, regardless of the shape of the contracted muscular passage, the contracted blood vessel, or the dilated artery. The present invention also relates to a method of producing such flexible self-expandable stents.
2. Description of the Related Art
Generally, blood vessels, in particular, arteries may be contracted at a part thereof due to thrombus, arteriosclerosis or the like to have angiostenosis, and may be dilated at a part thereof due to senility and/or some diseases with an aneurysm which is the arterial dilatation.
A surgical operation has been typically performed for treating the contracted artery or repairing the arterial dilatation through an artificial vessel replacement in which the contracted or dilated artery is replaced with an artificial blood vessel or through angioplasty. However, since the surgical operation for treating the contracted artery or repairing the arterial dilatation through the artificial vessel replacement or the angioplasty must be accompanied by making a large incision in the skin of a patient around the contracted or dilated artery, the operation undesirably leaves a large, ugly scar on the skin and mars the appearance of the patient. The surgical operation is also problematic in that it does not accomplish a desired operational effect.
The above-mentioned problems, experienced in the surgical operation for treating the contracted artery or repairing the arterial dilatation, are also caused in the artificial vessel replacement or the angioplasty to treat the stenosis of the gullet, the gall duct, the urethra, the formation of the artificial passage in the jugular vein, and the stenosis and blockade of the internal organs.
In an effort to overcome these problems, a variety of techniques to simply treat the contracted parts or repair the arterial dilatation, without surgery, have been proposed. One of the proposed techniques is the use of self-expandable stents made of a shape memory alloy.
The conventional self-expandable stent, used for insertion in a desired part of a contracted muscular passage, a contracted blood vessel, or an artery having an aneurysm to open the contracted part or repair the arterial dilatation, comprises a cylindrical stent body which is fabricated by knitting a plurality of shape memory alloy wires with each other to form a net-like hollow cylindrical body having a predetermined length and a plurality of diamond-shaped meshes.
To prevent an infiltration of tumor cells into the interior of the self-expandable stent and prevent undesired contact of a material, such as food, with the lesion, and allow the stent to reliably repair the arterial dilatation, the cylindrical stent body is provided with a coat layer on the external surface thereof to externally cover the sidewall of the stent body.
In such a case, the coat layer may be formed on the stent body by immersing the stent body in polytetrafluoroethylene (PTFE).
To place the self-expandable stent in a desired part of a contracted muscular passage or a dilated artery, the hollow cylindrical stent body is considerably reduced in volume by compressing the meshes of the stent body, and is inserted into the desired part of the contracted muscular passage or the dilated artery by using a stent inserting device, such as a catheter. When the compressed stent body is placed in the desired part, the stent body made of the shape memory alloy wires restores its original shape, thus opening the contracted part or repair the arterial dilatation.
However, the conventional self-expandable stents are problematic in that the stents are not preferably used in the bent parts of the contracted muscular passages or the dilated arteries, so that the usability of the stents is reduced.
That is, when the self-expandable stent is inserted into a bent part of a contracted muscular passage or a dilated artery, the stent does not maintain a desired bent shape corresponding to the bent part of the contracted muscular passage or the dilated artery, but restores its horizontal or vertical straight shape, thus lengthening the contracted or dilated part of the muscular passage or the artery and deforming the bent shape of the contracted or dilated part into a horizontal or vertical straight shape. The muscular passage or the dilated artery may be deformed to be narrower than its original size due to the stent, thereby hindering the circulation of a material, such as food or blood, and deteriorating the function of the stents.
In an effort to overcome the above-mentioned problems experienced in the conventional expandable stents, the inventor of the present invention proposed a flexible self-expandable stent which maintains a desired bent shape thereof corresponding to a desired part of a contracted muscular passage or a dilated artery, as disclosed in Korean Patent No. 2001-180245. As shown in FIGS. 1 to 3 of the accompanying drawings, the flexible self-expandable stent disclosed in Korean Patent No. 2001-180245 comprises a hollow cylindrical stent body which is fabricated by knitting first and second super-elastic shape memory alloy wires 10 and 11 to make a net-like structure 50 in that the first wire 10, which is zigzagged with a diagonal length P in a longitudinal direction of the stent body, is interlocked with the second wire 11, which is zigzagged with a diagonal length 2P in the longitudinal direction, at different positions to form a plurality of interlocked points 60 capable of allowing the stent body to contract and expand in the longitudinal direction. The net-like structure 50 also has a plurality of intersecting points 70 which are formed by a repeated intersection of the first and second wires 10 and 11 at a plurality of positions between the interlocked points 60. The intersecting points 70 allow the stent body to apply a force against the longitudinal contraction of the stent body. In the net-like structure 50, the interlocked points 60 and the intersecting points 70 define a plurality of diamond-shaped meshes 20.
The first and second wires 10 and 11 are thus interlocked with each other to be prevented from being separated from each other while allowing the stent body to somewhat freely contract and expand.
The wires 10 and 11 are produced by using a shape memory alloy through the steps of producing wires by shaping an alloy of harmless metals into a desired shape, and heat-treating the wires to allow the wires to restore the original shapes thereof at a predetermined temperature.
In such a case, the heat treatment for the shape memory alloy wires 10 and 11 is preferably performed at about 350° C.˜600° C. for 8˜30 min, as disclosed in Korean Patent No. 2001-180245.
The flexible self-expandable stent is preferably fabricated by using two shape memory alloy wires 10 and 11 each having a diameter ranging from 0.1 mm to 0.5 mm. When the diameter of the wires 10 and 11 is less than 0.1 mm, the wires 10 and 11 only have insufficient elasticity, so that the stent cannot effectively open a contracted or dilated part of the muscular passage or the artery. When the diameter of the wires 10 and 11 exceeds 0.5 mm, the meshes 20 of the stent body do not have sufficient space, so that it is almost impossible to reduce the volume of the stent to a desired level when the stent is inserted into a desired part of the contracted muscular passage or the dilated artery.
In the flexible self-expandable stent, it is necessary to set the number of bent parts 30, provided at each end of the stent body, to 3˜12. When there are more than 12 bent parts 30 provided at each end of the stent body, the size of the diamond-shaped meshes 20 is greatly reduced regardless of the diameter of the wires 10 and 11, so that it is almost impossible to reduce the volume of the stent body to a desired level when inserting the stent into a desired part of a contracted muscular passage or a dilated artery. When the number of the bent parts 30 provided at each end of the stent body is less than three, it is possible to reduce the volume of the stent body to a desired level when inserting the stent into the desired part of the contracted muscular passage or the dilated artery. However, in such a case, the stent has insufficient elasticity, so that the stent may fail to restore its original shape after the stent is inserted into the desired part of the contracted muscular passage or the dilated artery. Therefore, it is preferred to have between 3 and 12 bent parts 30.
The flexible self-expandable stent is placed in a desired part of a contracted muscular passage or a dilated artery in a manner similar to that described for the conventional stents. In addition, when the flexible self-expandable stent is placed in the contracted or dilated part of the muscular passage or the artery, the stent maintains its shape corresponding to the shape of the contracted or dilated part, regardless of a horizontal straight shape, a vertical straight shape or a bent shape of the contracted or dilated part, as shown in FIG. 4. The stent is thus effectively used for opening the contracted part or repairing the arterial dilatation, without deforming the shape of the contracted muscular passage, the contracted blood vessel, or the dilated artery.
In the flexible self-expandable stent, the net-like structure 50 of the cylindrical stent body has high flexibility which allows the stent body to be easily shaped as desired in response to an external force and maintain the shape in the contracted or dilated part of the body, since the net-like structure 50 is produced by knitting the wires 10 and 11 with each other to form the interlocked points 60, and the diameter φ of the stent body is set to a range which gives the desired elasticity to the stent body.
However, the flexible self-expandable stent is problematic in that the stent may not prevent an infiltration of tumor cells into the interior of the stent, or prevent undesired contact of a material, such as food, with the lesion, or repair the arterial dilatation. Therefore, it is necessary to form a coat layer on the external surface of the stent body to externally cover the sidewall of the stent body.
When the coat layer is formed on the external surface of the stent body to externally cover the sidewall of the stent body, the coat layer is integrated with the stent body into a single structure. In such a case, the flexible self-expandable stent may not take the desired shape in response to an external force or maintain the shape in the contracted or dilated part of the body.